A variety of endless chain type trenchers have heretofore been utilized. Normally, such trenchers comprise an elongated frame having one end thereof pivotally mounted on a vehicle for movement in a vertical plane parallel to the path of travel of the vehicle. The digging frame provides a mounting for a power-driven endless chain carrying a plurality of spaced digging scoops. Thus the depth of the trench to be dug is determined by the angular position of the digging frame with respect to the horizontal, hereinafter called the digging frame angle. Such angular position can be determined by hydraulic cylinders appropriately connected between the digging frame and the vehicle. If the vehicle is moving over perfectly level ground, sighting bars can be located along the path of the trench to be dug and a sighting bar provided on the digging frame adjacent to the operator's position so that whenever the operator's sighting bar departs from the reference plane defined by the stationary sighting bars, the operator may operate the hydraulic cylinders to change the angular position of the digging frame and thus re-establish the effective depth of the trench at the desired level; however, few if any trenching jobs involve perfectly level terrain and most prime mover vehicles which are employed to mount endless chain type diggers have relatively short coupled wheels. Thus, any obstructions or depressions in the terrain traversed by the vehicle will result in pitching movement of the vehicle which result in variations in the angular position of the digging frame, hence in the effective depth of the trench being dug. There is the further complication that the upstanding mast upon which the operator's sighting bar is mounted will not normally move vertically exactly the same amount as the variation in the depth of the trench. This is due to the fact that the trenching frame is pivotally mounted to the vehicle and any upward or downward movement of such frame results in a tilting movement of any upstanding mast relative to the true vertical, hence introducing a distortion and inaccuracy when such sighting bar is employed as the sole means for maintaining the depth of the trench at the desired level beneath a reference plane determined by a plurality of stationary sighting bars disposed along the path of the trench being dug.
In my co-pending application Ser. No. 628,095 filed Nov. 3, 1975, of which this application constitutes a continuation in part, it has been proposed to automatically control the digging depth of an endless chain type trencher by establishing a reference plane over the working area in the form of a rotating laser beam and then providing sensors carried by an upstanding vertical mast mounted on the digging frame to detect any departures of the desired depth of the trench relative to the reference plane and effect the necessary correction in the digging angle of the trencher. This equipment, while very effective, is necessarily expensive and the operator who does not have a large volume of trenching jobs may well prefer to utilize manual control of the digging depth of the trencher if he can be assured that reasonable accuracy is obtainable.